Saudi Crown Prince Sultan bin Abdul Aziz has died, the palace announced Saturday, leaving his brother Prince Nayef the likely successor to the ailing King Abdullah at a time of Middle East turmoil.

File picture shows Saudi Prince Sultan bin Abdul Aziz in Riyadh. The Saudi Crown Prince has died, a palace announcement on state television said, throwing his brother Prince Nayef into line to succeed King Abdullah as leader of the key oil producer. (AFP Photo/)

The crown prince, aged 80 according to government records, served as the oil kingpin\'s defence minister for nearly five decades but had been in the United States since mid-June for medical treatment. He was operated on in July.

A half brother of King Abdullah, Sultan spent long periods abroad for undisclosed medical treatment.

A Western diplomat, speaking to AFP on condition of anonymity, said Prince Sultan had been hooked up to life support systems at New York Presbyterian hospital and was declared "clinically dead" more than a month ago.

Condolences flooded into the kingdom from world leaders as news of the prince\'s death was confirmed.

US Secretary of State Hilary Clinton, on a trip to the Central Asian nation of Tajikistan, said the prince would be "missed," stressing Washington\'s enduring ties with the oil-rich Gulf state.

"I offer my deepest condolences for this loss to King Abdullah and the Saudi people," she said. "He will be missed."

In Jordan, King Abdullah II opened a World Economic Forum in the country\'s Dead Sea resort town with a minute of silence in the late prince\'s honour, hailing him as a "champion of the Arab and Muslim cause."

Britain\'s Prime Minister David Cameron said he was "saddened" to hear of Sultan\'s death.

"He had many friends in this country, and we have all benefited from his wisdom and expertise in international affairs over his long years of service," the prime minister said.

Prince Charles, the heir to the British throne, wrote to King Abdullah, his Clarence House office said.

"The Prince of Wales sent a personal letter of condolence to the king of Saudi Arabia expressing his deep sadness at the news," a spokesman said.

Syrian President Bashar al-Assad, despite strains in ties with Riyadh over his security forces\' deadly crackdown on anti-regime protests, also sent condolences to the king.

Sultan died while King Abdullah, himself 87, remains in hospital in Riyadh, a week after he had an operation on his back. State television aired footage Wednesday of King Abdullah in hospital as he received royal dignitaries.

The king underwent an operation in November for a debilitating herniated disc complicated by a haematoma, or blood clot, that put pressure on his spine, and he underwent further surgery to repair several vertebrae.

The latest operation was aimed at repairing a slackening ligament near the third vertebra. State media said the surgery was successful.

The advanced age and failing health of the king and of his half-brothers in line to succeed him have raised concerns about the future of the oil giant in the face of the turmoil rocking the Arab world.

The Al-Saud family has ruled Saudi Arabia ever since the kingdom\'s establishment in 1932 and under its rules of succession the throne passes in turn to the sons of its founder Abdul Aziz, all of whom are now elderly.

Prince Nayef, who is expected to become crown prince, is 78 and was named second deputy premier in March 2009, filling a potential void in the succession lineup.

Another half-brother of King Abdullah, he holds the interior portfolio.

Except for a few demonstrations by the Shiite minority in eastern Saudi Arabia, the Sunni-dominated absolute monarchy has been largely spared the wave of pro-democracy protests sweeping other Arab countries.

But it has felt increasingly threatened by the growing influence in the region of its Shiite arch-rival Iran, which Riyadh accuses of sowing sectarian tensions.

In March, Riyadh spearheaded a military intervention by Sunni-ruled Gulf states in Bahrain as its Sunni ruling family crushed month-long pro-democracy protests led by the island state\'s Shiite majority.

Prince Sultan\'s funeral is to be held on Tuesday after his body has been repatriated from the United States.

Sultan had battled colon cancer since 2004, going to Switzerland and then the United States to seek treatment, according to diplomats.

His long illness and absences abroad held up important government decisions while raising questions about how the monarchy would take shape in the next generation of the Al-Saud family.